FIG. 1 shows an "on-line" "non-linear" video editing system 10. One or more linear video sources 12, such as video tape recorders (VTRs), each output an analog or digital video signal to a compressor 14. The compressor 14 compresses each video signal and stores the digital compressed video signals in a non-linear storage device 16, such as a magnetic disk. Unlike the linear video signal source 12, which accesses video signals sequentially, the non-linear storage device 16 supports random access of video signals. Compression of the video signals in the compressor 14 may be achieved according to the MPEG, MPEG-2, Dolby.TM. AC-3, etc., standards for compressing video and associated audio. See ISO/IEC IS 13818--1, 2, 3: Generic Coding of Moving Pictures and Associated Audio: Systems, Video and Audio. The compressed video signals are then edited using a computer 18 which accesses the compressed video signals for display and editing. In displaying a compressed video signal, the non-linear storage device 16 outputs a compressed video signal to a video decompressor 19, which decompresses the video signal for display on a display monitor. After editing, the edited compressed video signal may be retrieved from the non-linear storage device 16 and decompressed by decompressor 19 for storage on a linear storage device (such as a VTR) or transmitted.
The non-linear video editing system 10 enables the operator to access the video in a random fashion instead of a sequential fashion. This facilitates the editing of the video and provides for rather precise editing. The editing is said to be "on-line" in that the final edited video event is incrementally constructed as the edits are made. For example, if the operator chooses to splice a first video clip to a second video clip, then the compressed form of the first video clip is spliced to the compressed form of the second video clip, at the time the operator makes such a choice to produce an edited compressed video signal. The edited video event is formed by decompressing the edited compressed video signal.
FIG. 2 shows a an "off-line" "non-linear" video editing system 50. Multiple linear video sources 12 are used to input video signals to a video compressor 14. The video compressor 14 compresses each video signal and stores the compressed video signal on a non-linear storage device 16. As before, a computer 18 can access the compressed video signals on the non-linear storage device 16 for viewing and editing. The compressed video signals are decompressed in decompressor 19 prior to viewing on a display monitor (not shown).
Unlike the video editing system 10, the video editing system 50 does not incrementally form the final edited video event as each edit decision is made. Rather, the formation of the final edited video event is deferred until the editing of the video signals is complete. The computer 18 generates a "edit decision list" as each editing operation is performed by the operator. Each edit decision on the edit decision list may, for example, indicate the kind of edit transition/operation to use (cut, fade, wipe dissolve, over-dub, etc.), which video clips are operated on by the edit operation and the edit points (e.g., specific pictures or audio frames at which the editing operation begins and ends) within those clips. The operator can preview a "facsimile" of the final event as formed by performing the edit operations indicated on the edit decision list on the compressed video signals stored in the non-linear storage device 16. After the operator has approved the edit decisions, the computer 18 accesses the original video signals stored on the linear storage devices 12 and performs edit operations on these video signals according to the edit decision list.
Typically, an off-line video editor 50 produces a final edited video event with better video and audio quality than an on-line video editor 10. This is because the video event produced by on-line video editing contains compression artifacts produced by the lossy or imperfect compression and decompression of the video. Nevertheless, an on-line video editor 10 is less expensive than an off-line video editor 50 because the on-line video editor 10 needs only a single linear storage device 12. Furthermore, the linear storage device 12 used in the on-line video editor 10 need not be as sophisticated as the linear storage device 12 used in the off-line video editor 50 since it does not have to advance to multiple specific edit points (as would be necessary in the off-line video editor in forming the final edited video event from the edit list). The on-line video editor 10 is easier to use because the physical media (video tapes) on which the original video information is stored need not be resident in the linear storage device 12 to create the final edited video event. This is because once the video signals are transferred to the non-linear storage device 16, the video signals need not be retrieved from the linear storage device again.
In order to reduce the level of compression artifacts present in the final edited video event in the on-line video editor 10, the on-line video editor 10 may typically use high compression data rates, high resolution and/or inter-frame coding. Video signals from source 12 in system 10 are compressed before being stored in the storage device 16 in order to save disk space and because the throughput of inexpensive disk drives is generally insufficient to support uncompressed high resolution video. To insure sufficiently high quality edited video, a non-linear editor typically uses high resolutions such as 720.times.480 pixels at 30 frames/sec and high compressed bit rates such as 18-50 Mbits/sec.
The use of high resolutions, high bit rates and/or inter-frame encoding in the compressor 14 can increase the difficulty of processing functions such as accessing stored compressed video streams, playing back more than one bit stream at the same time, and decoding/decompressing with trick modes such as fast forward and fast reverse. A compression system which utilizes compressed video bit streams having low resolution, low bit rate and/or only intra-frame encoding does not suffer these drawbacks. It is therefore desirable in many applications to provide a system in which multiple resolution and/or multiple bit rate versions of a given video signal can be compressed and stored. The high resolutions, high bit rates and inter-frame encoding can then be utilized when necessary, while the advantages of low resolution, low bit rates and intra-frame encoding can also be provided in appropriate applications.
Video servers represent another application in which storage of multiple versions of compressed video bit streams is desirable. Such video servers are used to deliver video bit streams to end users over data communication networks. For example, a World Wide Web server may be used to deliver video bit streams to different end users over different types of lines, including plain old telephone service (POTS) lines, integrated services digital network (ISDN) lines, T1 lines and the like. A version of a given compressed bit stream that may be suitable for a POTS user would be considered poor quality by a T1 user, and a bit stream suitable for a T1 user would be at too high a bit rate for a POTS user. It is therefore desirable for the video server to store a given video bit stream at multiple bit rates. The "optimal" resolution for a compressed video bit stream is the one that yields the best subjective video quality after decompression. This optimal resolution generally decreases with bit rate, such that it is desirable for the video server to compress the different bit rate streams at different resolutions.
FIG. 3 shows a conventional video compression system 20 which generates and stores multiple resolution versions of a given bit stream. The system 20 includes a video source 12, video compressor 14 and storage device 16 which operate in the manner previously described in conjunction with FIGS. 1 and 2. The system 20 also includes a video scaler 22 which receives a given video signal from the source 12 and generates a number of reduced resolution versions thereof. These reduced resolution versions are supplied to the video compressor 14, which generates a compressed video bit stream at an appropriate bit rate for each of the reduced resolution versions of the video signal, and stores the resulting compressed streams on the storage device 16.
The system 20 suffers from a number of significant problems. For example, each of the reduced resolution versions of a given video signal are separately and independently compressed. The information used to encode the video signal at one resolution is generally not used to facilitate the encoding process for the other reduced resolution versions. In addition, the video scaler 22 and video compressor 14 are not configured in a manner which enables them to share at least a portion of a common memory. The system 20 therefore requires relatively large amounts of memory. These and other drawbacks of the system 20 unduly increase its cost and complexity, and limit its usefulness in non-linear editor applications, video server applications and numerous other important video processing applications.
As is apparent from the above, there is a need for an improved multiple resolution video compression system in which hardware and processing resources can be shared to thereby significantly reduce the cost and complexity of the system.